4. Documentation requirements

When it is impossible or infeasible for a system or program to define a
particular behavior itself, it is permissible to state that the behavior
is unspecifiable and to explain the circumstances and reasons why this is so.

The implementation-defined items in the following list represent
characteristics and choices left to the discretion of the implementor,
provided that the requirements of this Standard are met. A system shall
document the values for, or behaviors of, each item.